Don’t be an enabler and buy it for it. Even if I didn’t believe anything was wrong with the clothes, if I knew they not within my parent’s standards, I would have felt they were fools if they’d bought them for me.
Neither of you should care what others say. You have to pay up the King of the World some day. He dressing however he feels he can be happiest, if it is within the conforms of HALACHA (it’s irrelevant if it is ‘kosher’ for the community’s mishugas) then he is doing the right thing and should not be bothered by what some other people say. Because they are wrong for caring more about the externals than they do about the Jew inside.
Number one rule I’ve learned to live by is don’t judge Judaism by the Jews. Let him realize that just because someone judges him doesn’t mean they are right, nor that they have the halachik rishut to do so. Explain to him that he has his own struggles (i.e. with conformity and shabbat, even if he doesn’t think they are worth working on) so perhaps he can understand that people in the community have struggles with their perceptions of others (and in the same way, they don’t either believe it is something worth working on), so he can associate with them!
He struggles with ben adam lamakom, they struggle with ben adam lechavero. He has to realize that when a Jew hurts another Jew, it is dead wrong on their part and he is fighting the wrong battle if he throws away the Torah because some other people give it a ‘bad reputation’.